1. Field of the Invention
The present invention relates generally to interleaving in an OFDM (Orthogonal Frequency Division Multiplexing) communication system, and in particular, to an apparatus and method for controlling AMC (Adaptive Modulation and Coding).
2. Description of the Related Art
With the introduction of a cellular mobile communication system in the U.S. in the late 1970's, Korea started to provide a voice communication service in a first generation (1G) analog mobile communication system, AMPS (Advanced Mobile Phone Service). In the mid 1990's, Korea deployed a second generation (2G) mobile communication system, CDMA (Code Division Multiple Access) to provide voice and low-speed data services.
In the late 1990's, Korea partially deployed a third generation (3G) mobile communication system, IMT-2000 (International Mobile Telecommunication-2000), aiming at advanced wireless multimedia service, worldwide roaming, and high-speed data service. The 3G mobile communication system was developed specifically to transmit data at high rate along with the rapid increase of serviced data amount.
Currently, the 3G mobile communication system is evolving to a fourth generation (4G) mobile communication system. The 4G mobile communication system is under standardization for the purpose of efficient interworking and integrated service between a wired communication network and a wireless communication network, beyond simple wireless communication service, which the previous-generation mobile communication systems provide. It follows that a technology of transmitting a large volume of data at a same capacity level available in the wired communication network must be developed for the wireless communication network.
In this context, studies are actively conducted on OFDM as a useful scheme for high-speed data transmission on a wired/wireless channels in the 4G mobile communication system. OFDM is a special type of MCM (Multi Carrier Modulation) in which a serial symbol sequence is converted to parallel symbol sequences and modulated to a plurality of mutually orthogonal sub-carriers (or sub-carrier channels).
In OFDM, the orthogonality between the sub-carriers enables optimum transmission efficiency in high-speed data transmission. Also, the robustness against multi-path fading further contributes to the optimum transmission efficiency. Frequency spectrums are overlapped, resulting in efficient frequency use and robustness against frequency selective fading and multi-path fading. OFDM uses a guard interval to thereby alleviate the effects of inter-symbol interference (ISI), enables simple designing of an equalizer, and is robust against impulse noise. These advantages have made OFDM widely used in high-speed, large-volume data communication systems such as IEEE (Institute of Electrical and Electronics Engineers) 802.16a and IEEE 802.16e.
FIG. 1 is a schematic block diagram of a transmitter in a conventional OFDM communication system. Referring to FIG. 1, the transmitter comprises an encoder 111, an interleaver 113, a symbol mapper 115, a serial-to-parallel converter (SPC) 117, a pilot symbol inserter 119, an IFFT (Inverse Fast Fourier Transformer) 121, a parallel-to-serial converter (PSC) 123, a guard interval inserter 125, a digital-to-analog converter (DAC) 127, and an RF (Radio Frequency) processor 129.
User data bits or control data bits to be transmitted are generated and provided to the encoder 111. The user data bits or control data bits are commonly called information data bits. The encoder 111 encodes the information data bits in a predetermined coding method such as convolutional coding or turbo coding having a predetermined coding rate. The interleaver 113 interleaves the coded bits in a predetermined interleaving method.
The symbol mapper 115 maps the interleaved bits to modulation symbols in a predetermined modulation method such as QPSK (Quadrature Phase Shift Keying), QAM (Quadrature Amplitude Modulation) or 16 QAM (16-ary QAM). The SPC 117 converts a serial modulation symbol sequence to parallel symbols. The pilot symbol inserter 119 inserts pilot symbols in the parallel modulation symbols.
The IFFT 121 performs an N-point inverse fast Fourier transformation on the signal received from the pilot symbol inserter 119. The PSC 123 serializes the IFFT symbols, and the guard interval inserter 125 inserts a guard interval in the serial symbols. The guard interval eliminates interference between an OFDM symbol transmitted in a previous OFDM symbol time and a current OFDM symbol to be transmitted in a current OFDM symbol time. The guard interval can be produced as a cyclic prefix or as a cyclic postfix. The cyclic prefix is created by copying a predetermined number of last samples of an OFDM symbol in the time domain and inserting them in an effective OFDM symbol, while the cyclic postfix is created by copying a predetermined number of first samples of an OFDM symbol in the time domain and inserting them in an effective OFDM symbol.
The DAC 127 converts the digital signal received from the guard interval inserter 125 to an analog signal. The RF processor 129, including a filter and a front end unit, processes the analog signal such that it can be transmitted. The RF signal is transmitted via a transmit antenna.
FIG. 2 is a schematic block diagram of a receiver in the conventional OFDM communication system. Referring to FIG. 2, the receiver comprises an RF processor 211, an ADC 213, a guard interval remover 215, an SPC 217, an FFT (Fast Fourier Transformer) 219, an equalizer 221, a pilot symbol extractor 223, a channel estimator 225, a PSC 227, a symbol demapper 229, a deinterleaver 231, and a decoder 233.
A signal transmitted from the transmitter illustrated in FIG. 1 experiences a multi-path channel and is received as a signal including noise by a receive antenna. The RF processor 211 downconverts the signal received from the receive antenna to an IF (Intermediate Frequency) signal. The ADC 213 converts the analog IF signal to a digital signal and the guard interval remover 215 removes a guard interval from the digital signal. The SPC 217 parallelizes the serial signal received from the guard interval remover 215 and the FFT 219 performs an N-point fast Fourier transformation on the parallel signals. The equalizer 221 channel-equalizes the FFT signal, and the PSC 227 serializes the equalized signal.
Meanwhile, the pilot symbol extractor 223 detects pilot symbols from the FFT signal and the channel estimator 225 estimates a channel using the pilot symbols and provides the channel estimation result to the equalizer 221. The receiver creates a CQI (Channel Quality Information) corresponding to the channel estimation result and transmits the CQI to the transmitter through a CQI transmitter (not shown).
The symbol demapper 229 demodulates the serial signal received from the PSC 227 in a demodulation method corresponding to the modulation method used in the transmitter. The deinterleaver 231 deinterleaves the demodulated symbols in a deinterleaving method corresponding to the interleaving method used in the transmitter. The decoder 233 decodes the deinterleaved symbols in a decoding method corresponding to the coding method used in the transmitter and outputs original information data bits.
As described above, the same transmit power and the same number of transmission bits are assigned to all sub-carriers, and a channel coding method is preset according to the transmission bits in the typical OFDM communication system. The signal transmitted from the transmitter reaches the receiver from multiple paths. Therefore, the received signal has experienced frequency selective fading. Although the transmitter transmits the signal on sub-carriers having the same transmit power and the same number of transmission bits, the receiver receives signals on the sub-carriers, which have different frequency responses due to the frequency selective fading. Accordingly, the channel decoder of the receiver corrects errors in the erroneous signal.
The above OFDM communication system uses a bit-interleaved coded modulation scheme. The bit-interleaved coded modulation scheme leads to a code diversity by which the performance of the receiver is improved as the fading coefficients of the sub-carriers are less correlated and fading changes fast. However, using a bit-interleaved coded modulation scheme results in a low frequency selectivity, that is, a less time-varying channel and a short channel delay spread in a radio environment as indoors, while it has a decreased error correction capability for a quasi-static frequency selective fading channel. Accordingly, the number of transmission bits on each sub-carrier is reduced or the total transmit power is increased to satisfy an error correction capability requirement set in the OFDM communication system. As a result, resource efficiency is decreased.